Silver halide color photography depends on the formation of dyes in order to reproduce an image. These dyes are typically formed from couplers present in or adjacent to the light sensitive silver halide emulsion layers which react to image light upon exposure. During development, the latent image recorded by the silver halide emulsion is developed to amplify the image. During this process in which silver halide is reduced to elemental silver, the color developer compound used is at the same time oxidized, as is typical in a redox reaction. The oxidized developer then reacts or couples with the coupler compound present in or adjacent to the emulsion layer to form a dye of the desired color.
Typically, a silver halide emulsion layer containing a magenta dye-forming coupler is sensitized to green light. This facilitates so-called negative-positive processing in which the image is initially captured in a negative format where black is captured as white, white as black, and the colors as their complimentary colors (e.g., green as magenta, blue as yellow, and red as cyan). Then the initial image can be optically printed in the correct colors on a reflective or transparent background through the device of optical printing which has the effect of producing a negative of the negative, or a positive image of the original scene.
Viewable images may also be produced through reversal processing in which the initial negative image is reversed by using a black and white developer, processed to remove the developed silver but leaving the undeveloped silver halide, and by then fogging the element in the presence of color developer to provide developed silver in inverse proportion to the amount of image light with corresponding dye formation.
For incorporation into a photographic element, the couplers are typically dissolved in high-boiling organic solvents known as "coupler solvents," and dispersed in gelatin with the aid of surfactants.
One of the difficulties with color couplers is achieving a desirable combination of physical and chemical properties of the coupler and the dye formed from it. For instance, the coupler should have good solubility in the coupler solvent, good dispersibility in gelatin, and high dye-forming activity. It should also have a high degree of resistance to decomposition due to light, heat and humidity, which can cause stains. The image dye should have the proper hue and a high degree of resistance to fading or hue changes due to light, heat and humidity.
Couplers that form magenta dyes upon reaction with oxidized color developing agent are described in such representative patents and publications as: U.S. Pat. Nos. 2,311,082, 2,343,703, 2,369,489, 2,600,788, 2,908,573, 3,062,653, 3,152,896, 3,519,529, 3,758,309, and 4,540,654; and "Farbkupplereine Literature Ubersicht," published in Agfa Mitteilungen, Band III, pp. 126-156 (1961). Typically, such couplers are pyrazolones and pyrazoloazoles, especially pyrazolo[2,3-b][1,2,4]triazoles described by Formula (A) and pyrazolo[3,2-c][1,2,4]triazoles described by Formula (B). ##STR1##
In Formulas (A) and (B), R and R.sub.2 represent substituents and Z is a hydrogen atom or a group capable of being split off during the coupling reaction. Typically, R.sub.2 is an alkyl group. An alkoxy group in this position leads to image dyes with very poor light stability.
The present invention is concerned with the pyrazolo[1,5-a]benzimidazole type of magenta dye-forming couplers (hereafter referred to as PBI couplers). These couplers may be described by Formula (1) ##STR2## in which R.sub.2 and R.sub.5 -R.sub.8 represent hydrogen or substituents and Z represents hydrogen or a group capable of being split off during the coupling reaction.
German patent 1,070,030 discloses PBI couplers which form magenta dyes upon coupling. In the examples given, R.sub.2 represents an alkyl or phenyl group. Couplers of these types are known to have poor coupling reactivity and to yield image dyes whose absorption spectra are too bathochromic for practical use in color photographic papers, and which have poor stability to light. International Patent Application WO 91/14970 describes PBI couplers with specifically substituted alkylthio coupling-off groups, including carboxyalkylthio groups. Such couplers offer marked improvements in coupling reactivity but do not offer improved dye hue or light stability. U.S. Pat. No. 5,143,821 describes PBI couplers in which R.sub.2 represents an alkoxy group. Such couplers are advantageous because they have much better coupling reactivity than those in which R.sub.2 represents an alkyl group and because the image dyes formed from them have good spectral absorption characteristics. Moreover, the dyes from these couplers have better light stability than the dyes from PBI couplers in which R.sub.2 is an alkyl group.
However, most PBI couplers are unstable to heat and humidity and have a high propensity to generate yellow stains during post-process keeping, known in the art as "yellowing."
A problem to be solved is to provide a dye-forming photographic element that resists formation of stains during keeping after processing or, in other words, an element that exhibits improved post-process keeping.